polyester allergy

If she didn't get the allergy from you, get a sample of the fabric and take it to fabric shops -- if there are fabric shops in your area. (Knowing what I do about allergies, I'd say keep it in a plastic bag when not in use. No use tempting fate by un-necessary exposure.)

Most mail-order fabric shops will send swatches on request.

If the concert gowns are black, that will help a lot, but there *are* different shades of black.

The way the fabric hangs and moves is also important. More details as to what the fabric you are trying to match is like would make it possible to give better suggestions.